In recent years, the level of particulate matters such as PM2.5 in the air has actively been measured. The concentration of particles in a gas is represented by a mass of particles contained in a gas of unit volume (for example, mg/m3 or μg/m3). This particle concentration is called a mass concentration. One of classic techniques for measuring the mass concentration of PM2.5 traps particles in a gas on a filter, and then measures the mass of the trapped particles. Alternatively, a beta attenuation measurement has been known as a technique that can measure a mass concentration automatically. The concentration obtained through the filter technique or the beta attenuation measurement is a mass concentration. Thus, the concentration of PM2.5 is currently normally indicated by the mass concentration. Furthermore, as a simple method, there is a light scattering detecting method that illuminates particles in a gas and then measures the number of particles contained in a gas of unit volume (e.g., the number of particles/m2) with the scattered light. There has been known several methods of automatically measuring the concentration of particles in the air as disclosed in Japanese Patent Application Publication Nos. 2001-343319, 2014-240733, and 2014-228309.